When a voltage is applied to an organic electroluminescence device (hereinafter, occasionally referred to as an organic EL device), holes are injected from an anode into an emitting layer and electrons are injected from a cathode into the emitting layer. The injected electrons and holes are recombined in an emitting layer to form excitons. Here, according to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%. In the classification according to the emission principle, in a fluorescent EL device which uses emission caused by singlet excitons, the limited value of an internal quantum efficiency of the organic EL device is believed to be 25%. On the other hand, in a phosphorescent EL device which uses emission caused by triplet excitons, it has been known that the internal quantum efficiency can be improved up to 100% when intersystem crossing efficiently occurs from the singlet excitons.
A technology for extending a lifetime of a fluorescent organic EL device has recently been improved and applied to a full-color display of a mobile phone, TV and the like. However, an efficiency of a fluorescent EL device is required to be improved.
Based on such a background, a highly efficient fluorescent organic EL device using delayed fluorescence has been proposed and developed. For instance, an organic EL device using TTF (Triplet-Triplet Fusion) mechanism that is one of mechanisms for delayed fluorescence has been proposed. The TTF mechanism utilizes a phenomenon in which singlet excitons are generated by collision between two triplet excitons.
By using delayed fluorescence by the TTF mechanism, it is considered that an internal quantum efficiency can be theoretically raised up to 40% even in fluorescent emission. However, as compared with phosphorescent emission, the fluorescent emission is still problematic on improving efficiency. Accordingly, in order to enhance the internal quantum efficiency, an organic EL device using another delayed fluorescence mechanism has been studied.
For instance, TADF (Thermally Activated Delayed Fluorescence) mechanism is used. The TADF mechanism utilizes a phenomenon in which inverse intersystem crossing from triplet excitons to singlet excitons is generated by using a material having a small energy gap (ΔST) between the singlet energy level and the triplet energy level. Thermally activated delayed fluorescence is described in, for instance, “Device Physics of Organic Semiconductor” Chihaya Adachi, pages 261-262, Mar. 22, 2012, published by Kodansha Company Ltd.
An organic EL device using the TADF mechanism is disclosed in, for instance, non-Patent Literature 1.
Non-Patent Literature 1 describes that green emission by the TADF mechanism can be efficiently obtained by using as a luminescent material a compound (hereinafter, occasionally abbreviated as PXZ-TRZ) having phenoxazine as an electron donating unit and 2,4,6-triphenyl-1,3,5-triazine as an electron acceptor unit. Non-Patent Literature 1 also describes that an organic EL device including an emitting layer in which PXZ-TRZ (luminescent material) is doped in CBP (4,4′-Bis(N-carbazolyl)-1,1′-biphenyl)(host material) emits light at an external quantum efficiency (EQE) of up to 12.5%.